The present invention generally relates to a semiconductor memory device, and, more particularly, to a redundant decision circuit for specifying a redundant memory cell in a memory cell array when a normal cell is defective.
To improve yield, a semiconductor memory device includes a redundant function that replaces a defective cell with a redundant cell in a cell array in the manufacturing process. For the redundant function, a redundant decision circuit is provided which generates a redundant signal in accordance with the cutting of a fuse of the defective cell. FIG. 1 is a schematic block diagram of a first conventional redundant decision circuit 100. A one-shot pulse generation circuit generates a control signal .phi.1 having a power supply VDD level after a predetermined time has elapsed from the start-up of the power supply VDD. The control signal .phi.1 is supplied to the gate of a P-channel MOS transistor Tr1. The source of the transistor is connected to the power supply VDD, and the drain (a node N1) is connected to a power supply Vss via a fuse 2. The node N1 is connected to an input terminal of a hold circuit 3. The hold circuit 3 supplies complementary redundant signals R, /R to a row decoder (not shown) in accordance with a potential at the node N1 after power-on. For example, when the redundant signal R goes high and the redundant signal /R goes low, the row decoder stops access to the defective cell in a memory cell array and accesses the previously specified redundant cell.
FIG. 2 is a waveform diagram of the power supply and control signal in the first conventional redundant decision circuit 100. When the power supply VDD is turned on, the control signal .phi.1 rises to the power supply VDD level after the predetermined delay time has elapsed from the start-up of the power supply VDD. The transistor Tr1 is turned on for a predetermined time t1 after the potential difference has become greater than the threshold and until the potential difference between the power supply VDD and control signal .phi.1 becomes smaller than a threshold of the transistor Tr1.
When the fuse 2 is not cut, the drain current of the transistor Tr1 flows in the power supply Vss via the fuse 2, and the node N1 is maintained at substantially the low potential power supply Vss level. The hold circuit 3 maintains the node N1 at an L level and outputs a redundant signal R Low and a redundant signal /R High. The hold circuit 3 maintains the output states of the redundant signals R, /R even if the transistor Tr1 is turned off after the predetermined time t1 has elapsed.
When the fuse 2 is cut, the node N1 rises to substantially the power supply VDD level for the predetermined time t1. Hereupon, the hold circuit 3 outputs the redundant signal R High and the redundant signal /R Low. The hold circuit 3 maintains the output states of the redundant signals R, /R even if the transistor Tr1 is turned off after the predetermined time t1 has elapsed.
In the first conventional redundant decision circuit 100, as shown by dotted lines in FIG. 2, when the start-up of the power supply VDD supplied to the source of the transistor Tr1 is slow under the cut state of the fuse 2, the on time t1 of the transistor Tr1 is shortened. This is because the control signal .phi.1 rises before the potential difference between the gate and source of the transistor Tr1 becomes sufficiently greater than the threshold. Hereupon, a sufficient drain current does not flow to the transistor Tr1 and the potential at the node N1 does not rise sufficiently. As a result, although the fuse 2 is cut, the hold circuit 3 outputs the redundant signal R Low and the redundant signal /R High. Moreover, when a leak current flows in the cut fuse 2, the potential rise at the node N1 is further suppressed and the probability of causing an incorrect decision increases.
To overcome such a problem, the on time t1 of the transistor Tr1 could be prolonged by further delaying the rising edge of the control signal .phi.1. However, in this case, the penetration current flowing in the power supply Vss from the power supply VDD via the transistor Tr1 and the fuse 2 increases. Moreover, the delay time of the one-shot pulse generation circuit 1 is set using a MOS (metal oxide semiconductor) capacitance. Accordingly, to prolong the delay time, the capacitance needs to be increased. However, a large MOS capacitance increases the circuit area of the one-shot pulse generation circuit 1.
FIG. 3 is a schematic block diagram of a second conventional redundant decision circuit 200. A control signal .phi.2 generated by a one-shot pulse generation circuit 4 is supplied to the gate of an N-channel MOS transistor Tr2. The source of the transistor Tr2 is connected to the power supply Vss, and its drain (a node N2) is connected to the power supply VDD via the fuse 2. An input terminal of the hold circuit 3 is connected to the node N2. When the redundant signal R goes low and the redundant signal /R goes high, the row decoder (not shown) stops access to the defective cell in the memory cell array and accesses the previously specified redundant cell.
FIG. 4 is a waveform diagram of the power supply and control signal of the second conventional redundant decision circuit 200. When the power supply VDD is turned on, after a predetermined time has elapsed from the start-up of the power supply VDD, the control signal .phi.2 having the power supply VDD level is generated. The transistor Tr2 is turned on for the predetermined time t2 until the potential difference between the power supply Vss and the control signal .phi.2 becomes smaller than the threshold of the transistor Tr2 after the potential difference has become greater than the threshold.
When the fuse 2 is not cut, a drain current is supplied from the power supply VDD to the transistor Tr2 via the fuse 2, and the node N2 is maintained at substantially the power supply VDD level in accordance with the resistance ratio between the fuse 2 and the transistor Tr2. The hold circuit 3 maintains the node N2 at the H level and outputs the redundant signal R High and the redundant signal /R Low.
When the fuse 2 is cut, the potential at the node N2 drops to substantially the power supply Vss level. Hereupon, the hold circuit 3 outputs the redundant R Low and the redundant signal /R High.
In the second conventional redundant decision circuit 200, when a leak current flows in the cut fuse 2, the potential drop at the node N2 is suppressed. Hereupon, although the fuse 2 is cut, the hold circuit 3 may output the redundant signal R High and the redundant signal /R Low.
To overcome such a problem, the on time t2 of the transistor Tr2 could be prolonged by further delaying the falling edge of the control signal .phi.2. However, in this case, the penetration current flowing in the power supply Vss from the power supply VDD via the fuse 2 and the transistor Tr2 increases. Moreover, to prolong the delay time of the one-shot pulse generation circuit 4, a MOS capacitance needs to be increased, which increases the circuit area of the one-shot pulse generation circuit 4.
It is an object of the present invention to provide a redundant decision circuit that prevents an incorrect decision when a fuse is cut, without increasing power consumption and circuit area.